This invention relates to a directional antenna system having side lobe cancellation. More particularly, it relates to an antenna system comprising a main directive antenna and a set of auxiliary directive antennas, connected in a passive network which uses the overall pattern of the auxiliary antennas to cancel part of the side lobe pattern of the main antenna.
To facilitate the explanation of this invention, we shall use the term "antenna" to include directive systems such as single element radiators combined with reflectors or arrays of single element radiators.
The invention is especially applicable to point-to-point microwave communication systems. To enhance the signal levels and also to avoid interference with others using the same frequencies, highly directive antennas are usually used for both transmission and reception between the stations in each link of the system. The radiation pattern of such an antenna includes a main lobe and a number of side lobes whose magnitude in relation to the main lobe and whose angular spacing depend on the effective aperture of the antenna, i.e. essentially its diameter in wavelengths.
The side lobes can present a serious problem if a transmitting station in another communications network using the same frequency is located nearby. Specifically, if the other station is in the direction of one of the side lobes in the pattern of a receiving station, the power received from that transmitting station may cause substantial interference with signals received from the desired principal lobe direction. Similarly, energy transmitted in a side lobe direction may interfere with the operation of a station in another network.
A number of schemes have been proposed for suppression of side lobes to minimize the effective amplitudes of interfering signals received from the directions of those lobes. For example, U.S. Pat. No. 3,982,245 describes an adaptive system including an omnidirectional antenna in addition to the directive main antenna. The signals from the two antennas are combined so that they interfere with each other in such manner as to materially reduce the strengths of signals received from the direction of a side lobe, without a corresponding reduction in the energy received from the direction of the principal lobe.
The system is characterized by cancellation of side lobe energy only over a narrow range of angles and a narrow range of frequency. However, because it is adaptive, it "tracks" the interfering signal and thus diminishes its effect even though the effective direction and frequency of that signal may vary. On the other hand, an adaptive arrangement requires active circuit elements and this diminishes its reliability. This can be a problem especially in networks in which the relay stations are remotely located and long periods of unattended operation are therefore desirable. Moreover, the system cancels side lobes only in the receiving mode. It can cancel side lobes during transmission only with substantial modification and, even then, only if there is occasional reception of signals from the direction in which cancellation is desired.